Trends in Rock Mechanics

Understanding the mechanical stability of rock and concrete structures is of utmost importance to the safe and efficient construction and operation of engineering projects. One of the most highlighted cases in recent years is the feasibility study for disposing of radioactive waste in deep underground repositories. Many international organisations are currently investigating this concept using underground laboratories as a !:ooi to provide development and testing of engineering design, stora methodologies and monitoring techniques. Acoustic techniques allow fracture growth around underground excavations to be observed and allow a quantification of the extent of damage and how this affects the physical properties of the rock. Current work involves integrating physical measurements from these techniques with numerical models to further understand the mechanics of the damage processes. It is proposed to then use these models in the repository design process to predict instabilities in the engineered structures that may occur over the repository's operational lifetime. Acoustic techniques will then provide remote monitoring capabilities that give a feedback mechanism by which these models can be updated and validated. In this paper we demonstrate the integration of acoustic monitoring and numerical modelling and discuss its future applicability to engineered structures in both rock and concrete.

This paper examines the effects of inelastic deformation of a rock-mass on the tunnel support loads predicted by the Einstein and Schwartz approximate design procedure. This procedure calculates the loads imposed on a circular support in a rock-mass subjected to non-uniform in situ stresses, under the assumptions that these stresses are `relaxed' (i.e., allowed to interact with the lining) after the support is installed, and that the rock-mass remains elastic. Results presented in this paper indicate that, for very deformable rock-masses i.e., rock-masses classified as `Very Poor' and those falling into the category of soils according to the Geological Strength Index (GSI) or the Rock-Mass Rating (RMR), the Einstein and Schwartz procedure over-predicts the loads and deformations on the support. Thus, for these cases the procedure is conservative. For relatively stiff rock- masses i.e., rock-masses of `Very Good' to `Poor' quality, the Einstein and Schwartz procedure under-predicts the loads and deformations on the support. For these cases the procedure is not conservative. It is also found that the support loads computed on the assumption that the rock-mass is subject to a uniform in situ stress, where this stress is the mean of the (actual) maximum and minimum stresses, provide useful practical insights into the loads on the support under non-uniform conditions.

This paper demonstrates the use of numerical modeling as an aid to assess the stability of large underground caverns and in determining appropriate remedial action during construction of two hydro-electric projects in India. One of the projects was in hard rock in western India, the other in soft formations of the Himalayas. Each project encountered design and construction problems. Numerical modeling is now considered an essential component of the design and construction phases of such hydroprojects.

The shear strength reduction technique is a method of performing slope stability analysis using a finite element or finite difference program. This technique is adapted here for use with the non-linear Hoek-Brown failure criterion, an empirical approach to estimating rock mass strength. Hoek-Brown strength parameters can be estimated using rock mass classification schemes such as RMR or GSI. Numerical results obtained by strength reduction are compared with limit analysis upper bound solutions derived using a series of linear failure surfaces tangent to the actual non-linear failure surface. Results are presented in graphs of normalized slope height versus RMR that could be used as stability charts.

There are cases that soft-ground tunnels are overlain by footings. The tunnel and footing may interact so that they may mutually influence the individual's stability adversely. Presently, there is no widely accepted analysis or design methodology available to ensure the stability of such footing/soft ground tunnel systems. As a step toward the development of such a methodology, this study was undertaken to investigate-the interaction between the soft-ground tunnel and the overlying strip footing. In this study, a two-dimensional plane-strain elasto-plastic finite element computer program was utilized to analyze the behavior of the tunnel for a wide range of soil type and tunnel conditions. Based on the results of the analysis, the mechanistic behavior of the footing/tunnel system was evaluated. It was concluded that the presence of the underground tunnels causes stress transfer from the tunnel crown to the spring line. The degree of stress redistribution depends on soil properties, tunnel location, tunnel size, and lining thickness. It was also concluded that for a given footing and tunnel condition, different lining thicknesses may be required for different soil types. Along a concrete lining, critical locations which required special attention in the lining design are in the intrados between crown and shoulder as well as between haunch and invert. This paper presents the results of the study and their practical applications.

The numerical approach to probabilistic key block analysis is used to examine the effect of joint dip angle on the sizes and numbers of key blocks in the 25-feet square tunnel. The three joint sets with identical angles between them are modeled on a rock mass through which the tunnel excavated. A series of tests are performed by varying the identical dip angles of the three joint sets in the increments of 22.5 degrees under two different spacing distributions. Results of probabilistic key block analysis indicate that joint dip angle has a profound influence on the key block size and the number of key block formed. The steeply dipping joints generate a greater number of large key blocks and create a larger key block size than do the shallowly dipping joints for both spacing distributions. With this knowledge, the range of sizes and numbers of key blocks, the essential elements for support design in a tunnel, can be predicted.

The influence of scale effects on the creep parameters of fractured rock masses are investigated through a series of in-situ and laboratory creep tests on slate and claystone. An in-situ creep test on slate indicates that deformation is highly heterogeneous, and that this resulting heterogeneous differential flow deformation is the primary cause for the failure. Creep tests on claystone define the magnitude of scale effects on the resulting creep parameters, including initial creep stress, long-term strength, deformation modulus, and viscosity, and also on the mode of ultimate failure. These parameters are correlated against specimen size, as indexed through the number of component blocks in the test specimens. These resulting empirical relations provide a defensible method for upscaling to field scale.

The moment tensor corresponding to acoustic emission (AE) source contains kinematical information on crack motion in a material. In order to determine the moment tensor components from AE waveforms, the SiGMA (simplified Green's functions for moment tensor analysis) procedure is developed, where AE sources can be located, classified into tensile cracks and shear cracks, and then the directions of crack motions are determined. In the present paper, the SiGMA procedure is further extended in rock-like materials, and a crack micrograpy is proposed. To this end, a relation between a damage parameter in damage mechanics and the moment tensor is clarified. Thus, damage evolution of the notched beam under bending is estimated from the trace components of the moment tensor. The scalar damage parameter is closely related with the crack density consisting of the crack volume. Therefore, the crack volumes are quantitatively estimated in uniaxial compression tests of plate specimens with a slit. In the linear elastic fracture mechanics (LEFM), the direction of crack extension can be determined from the concept of the maximumcircumferential stress Since the direction of crack motion can be derived from the eigenvectors of the moment tensor, the relation is applied to estimate the normalized stress intensity factors.

Hydraulic fracturing experiments were conducted by employing four 20cm-cubic granitic rock specimens of different grain size. Water was injected into a vertical hole in the specimen, bored normal to a rift plane along which micro cracks preferentially oriented. In the two horizontal directions, 12MPa and 6MPa confining pressures were applied to the specimen. Due to water injection, a horizontal crack along a rift plane was created in the specimens having larger grains, while a vertical crack along the direction of the maximum confining pressure was created in those having smaller grains. In addition, fault plane solutions of acoustic emission indicated that dominant micro fracturing mechanism was shear in the specimens having larger grains while it was tensile in those having smaller grains. These results demonstrate that grain size significantly affects the orientation of crack extension and the micro fracturing mechanism in hydraulic fracturing of granitic rocks.

A time domain moment inversion method has been developed for identification of internal damage mechanisms in rock. This method makes use of full waveforms, instead of first arrivals of signals received by NBS-type high-fidelity sensors to invert the damage mechanisms. The flat frequency response of NBS-type sensors enables us to yield signals comparable to mathematical solutions. This makes it possible to study the damage mechanism mathematically using the moment tensor inversion. Our current work includes conducting experiments on a Gypsum cube. Artificial acoustic sources made by piezoelectric elements are cast in the cube and used to simulate rock damages. By inverting the simulating signals, we could verify the validity of the time domain moment tensor inversion scheme.

Field observations indicate that failure of intact rock is associated with a rupture surface or shear band, where deformation is concentrated in a narrow zone; displacements occur with decreasing stress within the shear band, while outside the band the material remains more or less intact. When localization of deformation occurs within a rock structure, the global load-displacement behavior appears as a softening response. Therefore, understanding the initiation and propagation of the shear band is of fundamental concern, as this mechanism is associated with phenomena such as rock bursting and progressive failure. The study of localized failure under controlled conditions can be accomplished within a laboratory setting with the University of Minnesota Plane-Strain Compression Apparatus. An acoustic emission system was used to monitor the softening response of a sandstone in plane-strain compression. The recorded waveforms were analyzed for event location and source characterization. The complicated deconvolution process was eliminated by using a simplified calibration procedure. The moment tensors of the events associated with softening were evaluated, and the source mechanisms were determined. The sources of the post-peak events were characterized as being caused predominantly by slip in the direction of the global shear band. The displacement of each acoustic emission was estimated from the moment tensor by assuming a constant strain drop of 10^–4 over the slip area. Within a certain post-peak regime where 110 events were captured, the AE displacement was 0.011 mm, while globally 0.033 mm was measured.

Expansion of the Homestake Mining Companie's (HMC) Open pit in Lead South Dakota required the relocation of a state highway, several businesses and a number of residents. Historically, some areas surrounding the open pit have exhibited movement because of fluctuating groundwater levels, historic surface and underground mining and the variability of geologic strata. Expansion of the open pit toward the south encroached well into the City Limits of Lead South Dakota. The design called for excavation of the open cut down 800 ft at an overall slope angle of 48° adjacent to town. HMC worked with town officials designing the expansion to include a Visitors Center, Gift Shop and a new Town Park adjacent to the pit on the south rim. The expansion was designed to recover a significant amount of reserves that were down plunge from the main structure in an area that was previously mined using a variety of underground mining methods. Therefore, the bottom 350-ft of the 800-ft highwalls was expected to be founded in remnant stope backfill and caved rock. A movement monitoring system was installed shortly after completion of the rockfill buttress and the early stages of hard rock mining. Observed movements were evaluated using the FLAC (Fast Lagrangian Analysis of Continum) numerical modeling system to assess the risk of continued mining on the stability of the highwall and adjacent public facilities. Modeling indicated that the movements were within those anticipated. In fact, the mine plan was modified to extract additional reserves from this sensitive area by relocating the haul road. The FLAC model was used to predict the anticipated movement as well as assess the performance of the design modification during the mining process. The model allowed mine personnel to predict and evaluate the movements and provide a basis for assessment of stability. The community was further assured by the mine and the agencies that modeling indicated movements were within the anticipated levels and highwall stability was acceptable. Long-term prediction of movements was and will continue to be evaluated and compared to the model for closure of the open cut operation. This paper presents the basis for the model, assumptions in development of the model and the comparison of the predicted to actual movements observed. Proving that a FLAC model can be an excellent tool for the prediction of deformations in a project where deformation is a very sensitive issue.

The spring of 1997 marked the beginning of a professionally engineered, state-of-the-art remediation grouting program at the Toronto Transit Commission (TTC). A Tunnel Leak Remediation program was developed with the objective of stopping the water infiltration problems that plagued the Toronto subway tunnels since their construction. Water infiltration into the subway tunnels results in service affecting delays as well as concerns relating to the safe operation of a transportation system serving over a million riders per day. Problems include: accelerated aging of the rail and rail fastening systems, deterioration and malfunction of electrical systems and associated components and deterioration of the structure itself. A two-hour nightly working window, in which maintenance activities can be performed, combined with the marginally injectable ground conditions and difficult tunnel structure made achieving positive results especially challenging. An engineered solution grouting injection program using acrylamide technically proved to be the most suitable design for this particular application. The newly hired in-house work force had never previously worked with acrylamide solution grouts and therefore had to be trained and coached in all aspects of the grouting procedures. The acrylamide grouting operation is closely monitored under stringent quality control parameters and is implemented under what is likely one of the highest levels of personal protection ever used for an acrylamide grouting application. Acrylamide was never previously used anywhere on Toronto Transit Commission property, but it has demonstrated to be an invaluable tool in solving difficult water infiltration problems in difficult ground conditions under a limited two hour working window. This paper shares some of the experiences gained to date in this on-going Tunnel Leak Remediation Program at the Toronto Transit Commission.

This paper presents an overview of thirteen major tunnels in Colorado. The purpose of the paper is to document for easy reference the salient characteristics of the runnels for future reference and for comparison with potential future project. Included in the review of are: a summary of the ground conditions, runnel length and size, construction methods, support and lining types, and costs. Ail of the tunnels are situated in rock including shale, granite, gneiss, quartzite, and volcanic tuff. Review of these tunnels reveals changes in technologies few ground support and tunnel lining consistent with the industry. Additionally, it is noted that most of the tunnels used drill and blast excavation methods. However, if constructed with current technologies some of these tunnels, especially the long water tunnels, would likely be excavated with tunnel boring machines. Continued growth of Colorado's population with parallel needs for upgraded infrastructure in a mountainous area will likely result in more Colorado tunnels.